Docs: Add a "seealso" section to the module docs (#45949)

* Docs: Add a separate  "seealso" section to the module docs
to list related modules and/or related references. This clears up the notes
section for things that are actual notes.

So you can add a section in your module documentation and four types of
references are possible.

    seealso:

    # Reference by module name
    - module: aci_tenant

    # Reference by module name, including description
    - module: aci_tenant
      description: ACI module to create tenants on a Cisco ACI fabric.

    # Reference by rST documentation anchor
    - ref: aci_guide
      description: Detailed information on how to manage your ACI infrastructure using Ansible.

    # Reference by Internet resource
    - name: APIC Management Information Model reference
      description: Complete reference of the APIC object model.
      link: https://developer.cisco.com/docs/apic-mim-ref/

This PR also includes:

- Implements ansible-doc support
- Implements schema support for the seealso options
- Updates to the development documentation
- Rename filter convert_symbols_to_format to rst_ify, cfr the existing html_ify and tty_ify filters
  - This makes the existing template a lot easier to read and fixes the confusion I had myself rereading the template (again).
- We fixed the possible suboption types (which was limited to 'bool' only)

* Use latest stable instead of devel docs

test/sanity/validate-modules/schema.py

@@ -1,20 +1,8 @@
 # -*- coding: utf-8 -*-
-#
-# Copyright (C) 2015 Matt Martz <matt@sivel.net>
-# Copyright (C) 2015 Rackspace US, Inc.
-#
-#    This program is free software: you can redistribute it and/or modify
-#    it under the terms of the GNU General Public License as published by
-#    the Free Software Foundation, either version 3 of the License, or
-#    (at your option) any later version.
-#
-#    This program is distributed in the hope that it will be useful,
-#    but WITHOUT ANY WARRANTY; without even the implied warranty of
-#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-#    GNU General Public License for more details.
-#
-#    You should have received a copy of the GNU General Public License
-#    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Copyright: (c) 2015, Matt Martz <matt@sivel.net>
+# Copyright: (c) 2015, Rackspace US, Inc.
+# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
 
 import re
 from voluptuous import ALLOW_EXTRA, PREVENT_EXTRA, All, Any, Length, Invalid, Required, Schema, Self
@@ -47,6 +35,26 @@ def sequence_of_sequences(min=None, max=None):
     )
 
 
+seealso_schema = Schema(
+    [
+        Any(
+            {
+                Required('module'): Any(*string_types),
+                'description': Any(*string_types),
+            },
+            {
+                Required('ref'): Any(*string_types),
+                Required('description'): Any(*string_types),
+            },
+            {
+                Required('name'): Any(*string_types),
+                Required('link'): Any(*string_types),
+                Required('description'): Any(*string_types),
+            },
+        ),
+    ]
+)
+
 ansible_module_kwargs_schema = Schema(
     {
         'argument_spec': dict,
@@ -174,6 +182,7 @@ def doc_schema(module_name):
         Required('version_added'): Any(float, *string_types),
         Required('author'): All(Any(None, list_string_types, *string_types), author),
         'notes': Any(None, list_string_types),
+        'seealso': Any(None, seealso_schema),
         'requirements': list_string_types,
         'todo': Any(None, list_string_types, *string_types),
         'options': Any(None, *list_dict_option_schema),